In several primate species, adult males peacefully co-reside in the same social group and females are reported to mate with multiple adult males. Under these conditions, individual males and females may rely on a range of social strategies to increase reproductive opportunities. We investigated the mating behavior of Alouatta caraya on Isla Brasilera (27º 20' S-58º 40' W) in Argentina. Two groups were followed 5-days a month from sunrise to sunset from April 2003 to November 2004. We observed 216 copulations for 8 females (32% were extra-group copulations [EGC] and 62% intra-group copulations [IGC]), and 210 copulations involving 7 males (31% EGC, 69% IGC). Courtship, measured as the exchange of grooming before and after copulations was more frequent during IGC than EGC for males and females (G-test, p<0.01). EGC occurred most frequently outside of visual contact of a females’ group (G-test, p<0.01), but were interrupted if observed by a resident adult male. In contrast, IGC were never interrupted by resident males. Female howlers copulated with multiple males before, during and after conception (3.4±0.9 males/month during ovulation, 1.5±0.7 males/month during gestation, and 1.6±0.5 males/month during lactation). Social tolerance and affiliative behavior among resident males may promote cooperative group defense. Promiscuity in females may have several functions including mate choice, sperm depletion, extragroup male recruitment, paternal uncertainty and infanticide avoidance.